Diversion endangers Lake Manitoba's future

Whenever the Portage Diversion is opened, tonnes of phosphorus are dumped into Lake Manitoba. Photo Store

Two floods in the last four years have been devastating for the farmers, ranchers, First Nations, cottagers, and permanent residents around Lake Manitoba. The floods have also wrought ecological disaster. A brown ribbon of death now surrounds the water's edge where floodwaters destroyed the lakeside forest. And now we are killing the lake itself.

If we continue to routinely divert a major river system -- the Assiniboine -- from one watershed to another and continue to overload a lake ripe for eutrophication with phosphorus, we risk turning Lake Manitoba into the world's largest bowl of toxic blue-green Jell-O.

Flood Fight

Lake Manitoba is the world's 33rd-largest freshwater lake. It is exceedingly shallow with an average depth of just 4.5 metres. It is effectively the world's largest pond. Like most ponds, it warms quickly in summer and is biologically productive. It is now too productive.

Eutrophication occurs when a lake is overloaded with nutrients, especially phosphorus and nitrogen. In freshwater lakes, the key limiting nutrient is usually phosphorus. The most obvious results are blooms of blue-green algae (actually cyanobacteria). The spectacular blooms on Lake Winnipeg, ones that can be seen from space, earned it the dubious title of the most threatened lake in the world.

Phosphorus is oxygen to the fire of eutrophication: Cut off its supply and you stop the blooms. In Lake Manitoba, phosphorus is usually the nutrient in short supply. Except when the Portage Diversion is open.

Every time the diversion is opened, tons of phosphorus are dumped into a lake that does not need it. The diversion becomes the primary phosphorus source for the lake. The phosphorus that is carried in Assiniboine waters arises from various sources. Some is natural, some is from towns and cities upstream, some is from agricultural runoff. Phosphorus loading into Lake Manitoba is now a problem. A big problem.

In the summer of 2013, a fish kill spanned the lake, from Lundar on the east to Langruth on the west. Dead fish of all species and sizes littered the lakeshore. The most likely explanation was oxygen depletion associated with algal growth. This fish kill was deeply worrisome, potentially a leading indicator of more severe problems ahead.

Two recent reviews show phosphorus loading rises with the use of the Portage Diversion and report levels of biological productivity that have risen into the danger zone on Lake Manitoba.

In 2003, the Lake Manitoba Regulation Review provided the following clear recommendation to the minister of conservation: "The use of the Portage Diversion should be restricted to those periods of time and flows which are absolutely necessary to protect downstream interests along the Assiniboine River and in Winnipeg. The operating rules of the Portage Diversion should be re-examined, with the objective of asserting its primary function as a short-term flood-protection work, and to minimizing its discharge of nutrients, sediments, debris and other materials into Lake Manitoba."

This recommendation was echoed by the province's own scientists from Conservation and Water Stewardship in presenting to the Lake Manitoba Lake St. Martin Regulation Review after the 2011 flood. The conclusion was the Portage Diversion needed to be used less to protect water quality.

There is an obvious reason why.

Measurement of chlorophyll-a is used by water-quality scientists as an index of algal growth, a shorthand measure of eutrophication. In the 2013 review, it was reported that chlorophyll-a levels in Lake Manitoba were within the reported historical range. Nothing to worry about.

However, what was missed was the large blue-green elephant in the room tapping them on shoulder.

Chlorophyll-a levels were reported back to 1992. To use a term familiar to quantitative ecologists, a clear and significant time trend exists in the data.

Over the last two decades, as the Portage Diversion has been used more and more, chlorophyll-a levels have more than doubled. Average levels now rival those seen on Lake Winnipeg. And Lake Winnipeg has the highest chlorophyll-a levels of any large lake in the world, a big reason why it has been aptly described as the "sickest lake in the world."

Lake Manitoba has joined Lake Winnipeg in intensive care.

The 2003 review, and also the later 2013 review, recommended the Portage Diversion be used less for a variety of reasons including water-quality concerns. The response of the current provincial government, the recipient of both reviews? Use it more.

In 2013, flows on the lower Assiniboine were artificially restricted to just over 8,000 cfs with any surplus sent to Lake Manitoba in the Portage Diversion. Much more water was sent to Lake Manitoba than necessary. In 2014, the policy was to artificially restrict flows to 10,500 cfs. That was until all hell broke loose in late June.

This water-management policy drips with irony.

In early June, the province announced with great fanfare a progressive water-management strategy to restore the health of a very sick Lake Winnipeg by addressing the problem of eutrophication. That was excellent news.

But while Manitoba Conservation was rescuing Lake Winnipeg, one lake over, Manitoba Infrastructure and Transport was doing exactly the opposite. They were using the Portage Diversion and wrecking water quality on Lake Manitoba.

An ecological crisis has arrived on Lake Manitoba with this policy dissonance. Now it is not just one lake we need to rescue, but two.

Scott Forbes is an ecologist and professor of biology at the University of Winnipeg and has owned property on Lake Manitoba for 15 years.

This is the first of two columns by Forbes on the future of Lake Manitoba. The second will be published Tuesday.

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